In a conventional reversible pump-turbine, guide vanes are caused to close down with a pattern as shown by the dotted lines in FIGS. 11 (a) and (b) the time of generation mode emergency shutdown and pump mode emergency shutdown. Description will first be given for generation mode emergency shutdown. In this situation, the guide vanes are suddenly closed at the same time as a circuit breaker of an electrical generator is opened. When the initial open amount is Yt0, the guide vanes are closed to open amount Yt1 at a first restricted rate (rapid closure rate) previously set for use in the generation mode that has been, as shown by the dotted lines in FIG. 11(a) and from that point it is automatically switched to a second previously set restricted rate (closure rate after change) for use in the generation mode, and after that the guide vanes are closed at that rate. These first and second restricted rates are rendered by restricting an amount of passing oil in a control valve for a guide vane servo motor driving the guide vanes. If the opening amount becomes less than Yt2, a fully closed end cushion of the guide vane servo motor is actuated, which causes the closure rate to be lowered even further, as shown by the dotted lines in FIG. 11(a). The reason for automatically switching from the first restricted rate (rapid closure rate) to the second restricted rate (closure rate after change) in this way with the opening amount at Yt1 is described in detail in Japanese Patent Laid-open Publication No. Sho. 61-47981, etc., by the inventor of this invention. That is, as a result of raising and lowering the unit speed Ni after the circuit breaker is opened, the operating point trajectory of the pump turbine breaks into S0 to S1 and S1 to S3, the latter moving in a S0-called S shaped characteristic(s) of the pump turbine, as shown by the dotted line in FIG. 7, and because tracing of the S-shaped characteristics in a direction of reduced discharge is commenced there is a need to switch the guide vanes to a slow closure rate before the commencement. What is meant by the S-shaped characteristic here is the portion of dQ1/dN1&gt;0 as shown in FIG. 5.
Next, emergency shutdown in a pumping mode will be described. The guide vanes are also closed rapidly at the same time as the circuitbreaker is opened. One criteria for determining the guide vane closure pattern in this case will be described using FIG. 3. In the case where the guide vanes are closed rapidly, as shown by the solid lines in FIG. 3(b) the rotation speed is Na when the guide vanes become fully closed, while the rotation speed falls to Na' at the time of fully closed when the guide vanes is closed slowly, as shown by dotted lines in FIG. 3(b). FIG. 4 shows the relationship between time required for guide vanes to be fully closed and the rotation speed when the guide vanes are fully closed. As will be understood better from FIG. 4, if the time required to fully close the guide vanes becomes a long time, that is if the closure rate of the guide vanes is slow, Na' no longer has a positive value but has a minus value. What this actually means is that reverse rotation may arise in the pump mode. This reverse rotation is extremely damaging due to wear on a thrust bearing.
Another criterion determining a guide vane closure pattern at the time of emergency shutdown in the pump mode will be described using FIGS. 8(a) and 8(b).
This description will be given with X0 and Xa as guide vane opening amounts, X0&gt;Xa. A trajectory of the operating point when the guide vanes are closed rapidly is shown by the dotted line in FIG. 8(a). By way of comparison, the case when the guide vanes are closed slowly is shown by the dotted line in FIG. 8(b). Reverse discharge, that is, reverse flow, arises when rotation speed is positive, namely during pump rotation. Even though a reversible pump-turbine has pump rotation, the operating condition becomes to allow turbine flow as accompanied with oscillations and noises. The problem is here that if the guide vane opening amount is too large when reverse flow appears, the operating conditions become bad. Depending on the situation, the reversible pump-turbine can also become damaged. Even further a worse condition than in FIG. 8(b) is possible. If the guide vanes are closed more slowly reverse rotation also will occur even in the pump mode. The guide vanes will be fully closed before long no matter how slowly the guide vanes are closed, and so the operating points finally converge at (0, 0).
The guide vane closure pattern for pumping mode emergency shutdown as described above is determined not to allow reverse rotation, and so determined that reverse flow is suppressed to the minimum.
The above describes conventional technology relating to guide vane closure patterns, and on the other hand conventional technology relating to inlet valve closure patterns is shown in FIGS. 9(a) and 9(b). That is, with respect to an inlet valve, the maximum closure rate is determined as a rate close to the second control rate and lower than the first control rate for the guide vanes at the time of emergency shutdown in the generating mode, and this rate is used for both the generating mode and the pumping mode. That is, the rate at the time of generating mode emergency shutdown and the rate at the time of pumping mode emergency shutdown are made the same. Strictly speaking, the load torque acting on the inlet valve is different in the generating mode emergency shutdown and the pumping mode emergency shutdown, which means that, even if the inlet valve closing rate is set to the same rate at the time of a dry test, there are slight differences when the inlet valve is operated in water, but these can be ignored. A reversible pump-turbine that is the subject of the present invention generally carries out flow amount adjustment using the guide vanes, and so does not cover the case where usual discharge adjustment at the time of start-up etc. is carried out using an inlet valve. An inlet valve is considered which is to be opened prior to opening the guide vanes and to be closed after closing the guide vanes, and used in maintenance and inspection operations of the reversible pump-turbine.
However, in adopting the closure patterns of the above described conventional technology as closely as possible, it will be understood that the inlet valve can not achieve the desired back-up function at the time of pumping mode emergency shutdown. That is, consider the case of FIGS. 2(a) and 2(b) where the guide vanes have failed and can not be closed at all.
This can be understood on the basis of an extent to which the inlet valve, can carry out the water flow reducing function in place of the guide vanes. The inlet valve is closed in the pump mode at the same slow closure rate as in the generating mode,and naturally reverse rotation can not be avoided more or less. A typical example of the relationship between the time required to fully close the inlet valve and the rotation speed at the time when the inlet valve is fully closed will be described using FIG. 4. It is assumed that the guide vane closure pattern is as shown by the solid line in FIG. 3(b) and the inlet valve closure pattern is as shown by the dotted line in FIG. 3(b). In such a case, if it is assumed that the guide vanes have failed, the rotation speed at the time the inlet valve is fully closed is lowered to Na' and does not lead to reverse rotation. However, As shown in FIG. 4, if the time required to fully close the inlet valve is further prolonged the rotation speed at the time the inlet valve is fully closed is lowered rapidly, and if the time required to fully close the inlet valve becomes longer than Tax, reverse rotation becomes a possibility. In the above described conventional art, for example, even if reverse rotation has been avoided, a large amount of reverse flow is possible transiently during positive rotation, as shown in FIG. 8(b), and clearly the reversible pump-turbine is exposed to a dangerous condition. On the other hand, If the closure rate of the inlet valve is increased, the inlet valve is allowed to play a part in substantially throttling discharge at the time of generating mode emergency shutdown, and depending on the situation when the operation point of the pump turbine enters the S-shaped characteristics there is a possibility of abnormal water hammer.
The object of the present invention is to provide a safe system that eliminates the drawbacks of the above described related art, and enables the back-up function of discharge adjustment by the inlet valve at the time of emergency shutdown in the pump mode, but on the other hand does not have any harmful effect in the generating mode.